On-load tap changers have been known for many years; although they previously had mechanical contacts, which operate in oil, for load changeover, numerous more recent apparatus have vacuum switching tubes. The use of vacuum switching tubes for load changeover prevents formation of arcs in the oil and thus contamination of the load changeover switch oil. It thus allows lengthened maintenance intervals.
On-load tap changers with vacuum switching tubes are known with numerous different circuits with two, three, four or even more switching vacuum switching tubes per phase. A typical circuit, here with three vacuum switching tubes per phase, is shown by DE 10 2007 004 530 A1.
However, in various cases of use of such known on-load tap changers with vacuum switching tubes for regulation of power transformers a high surge-voltage strength of up to 100 kV and significantly above that is required. Such undesired surge voltages, the height of which is substantially dependent on the construction of the tapped transformer and the winding parts between the individual tap steps, are on the one hand lightning surge voltages that result from lightning strikes in the mains. On the other hand, switching surge voltages can also occur that are caused by unpredictable switching surges in the mains to be regulated. In the case of insufficient surge-voltage strength of the on-load tap changer a transient step short-circuit or undesired disruption of the ceramic or the damping screen of vacuum switching tubes in the load branch not conducting the load current can occur, which not only can cause long-term damage thereof, but also is generally undesirable. This leads in many cases in the design of the switching paths and thus particularly also the vacuum switching tubes to over-dimensioning so that they safely withstand the described voltage load. Not only the small constructional space currently available with modern apparatus, but also the economics as well as serviceability of such vacuum switching tubes have the consequence that such over-dimensioning appears to be disadvantageous.
In order to combat excessive surge-voltage loads it is already known from DE 2357209 A and DE 2604344 to provide protective spark gaps or voltage-dependent resistors between the load branches; however, these means are, in various cases, insufficient and are unable to exclude or completely exclude harmful surge-voltage loads in their effect.